1. Field of the Invention
The invention relates to a gas sensor, such as an oxygen sensor, HC sensor, NOx sensor, etc., which measures a gas introduced into the sensor and detects a component of the gas.
2. Description of the Related Art
Conventional gas sensors have a metal casing, inside which resides a detection element formed in a bar-like or cylindrical shape and having, at a tip, a detecting section to detect a component gas. For a gas sensor using air as a reference gas, e.g. an oxygen sensor, it is necessary to provide a portion for introducing air into the casing and accommodating a detection element or providing a filter support cylinder coupled to a rear end of the casing. This type of air introducing element can be formed, for example, by opening a gas passage hole in a rear end side wall of the casing or filter support cylinder and covering it with a water repellent filter. Meanwhile, an air introducing element may be provided by a lead wire insertion hole formed in a rubber grommet sealing lead wire extension port. This method introduces air into a casing through a gap formed between a coat material covering a lead-wire outer surface and a lead wire insertion hole.
In some conventional structures as described above, a ceramic separator formed with a lead wire insertion hole is provided in a casing (or filter support cylinder) in order to prevent a short circuit between lead wires extending from a detection element and heater. This ceramic separator is formed in a cylindrical shape matched to the casing, and is usually arranged at a rear end inside of the casing (or filter support cylinder) in order to be positioned on a side to extend a lead wire therefrom. On the other hand, an air introducing element as above is formed at the casing (or filter support cylinder) rear end, and a gap is provided between an outer peripheral surface and an inner peripheral surface of the casing (or filter support cylinder), in many cases, to provide a passage for the introduced air. However, recently there has been a movement toward reducing the size of the sensor, and, thus, there is an effort to reduce the diameter of the casing (or filter support cylinder) as small as possible. Thus, due to this effort to reduce sensor size, it is becoming difficult to provide a sufficient gap between the ceramic separator and the casing (or filter support cylinder). Although it is theoretically possible to reduce the diameter of the ceramic separator, there is a limitation on the ability to reduce the diameter of the ceramic separator size reduction on the assumption of securing and still maintain the basic function of the ceramic separator and prevent against shorting between the lead wires.
It is therefore an object of the invention to provide a gas sensor which can efficiently flow air through the casing while having a sufficient on-axis sectional area for a ceramic separator that allows lead wires to pass through.